<!DOCTYPE html>
<html>
<head>
<style>
.flex-container {
  display: flex;
  background-color: #f1f1f1;
}

.flex-container > div {
  background-color: DodgerBlue;
  color: white;
  margin: 10px;
  text-align: center;
  line-height: 75px;
  font-size: 30px;
}
</style>
</head>
<body>

<div class="flex-container">
  <div style="flex-grow: 1">1</div>
  <div style="flex-grow: 1">2</div>
  <div style="flex-grow: 3">3</div>
</div>
</body>
</html>